How to get only one Specific Country's States/Province in dropdown?

Question

I'm working on Magento 2.3.2, I override a Shipping Method file which is system.xml for adding custom fields. Here it is:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="carriers" type="text" sortOrder="320" showInDefault="1" showInWebsite="1" showInStore="1">
            <group id="flatrate" translate="label" type="text" sortOrder="0" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Flat Rate</label>

                <field id="tax_address" translate="label" type="text" sortOrder="98" showInDefault="1" showInWebsite="1" showInStore="0" >
                    <label>Tax Address</label>
                </field>
                <field id="tax_state" translate="label" type="select" sortOrder="99" showInDefault="1" showInWebsite="1" showInStore="0" >
                    <label>Tax State</label>
                    <source_model>Magento\Directory\Model\Config\Source\Allregion</source_model>
                </field>
                <field id="tax_zip_code" translate="label" type="text" sortOrder="99" showInDefault="1" showInWebsite="1" showInStore="0" >
                    <label>Tax Zip Code</label>
                </field>

        </section>
    </system>
</config>

In Tax State field I'm using <source_model>Magento\Directory\Model\Config\Source\Allregion</source_model> to get Regions/State/Provice, I know it'll fetch me all country regions, I just want to fetch only United Stated Regions.

This is the file Magento\Directory\Model\Config\Source which is responsible for getting All Regions:

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Directory\Model\Config\Source;

/**
 * Options provider for regions list
 *
 * @api
 * @since 100.0.2
 */
class Allregion implements \Magento\Framework\Option\ArrayInterface
{
    /**
     * @var array
     */
    protected $_countries;

    /**
     * @var array
     */
    protected $_options;

    /**
     * @var \Magento\Directory\Model\ResourceModel\Country\CollectionFactory
     */
    protected $_countryCollectionFactory;

    /**
     * @var \Magento\Directory\Model\ResourceModel\Region\CollectionFactory
     */
    protected $_regionCollectionFactory;

    /**
     * @param \Magento\Directory\Model\ResourceModel\Country\CollectionFactory $countryCollectionFactory
     * @param \Magento\Directory\Model\ResourceModel\Region\CollectionFactory $regionCollectionFactory
     */
    public function __construct(
        \Magento\Directory\Model\ResourceModel\Country\CollectionFactory $countryCollectionFactory,
        \Magento\Directory\Model\ResourceModel\Region\CollectionFactory $regionCollectionFactory
    ) {
        $this->_countryCollectionFactory = $countryCollectionFactory;
        $this->_regionCollectionFactory = $regionCollectionFactory;
    }

    /**
     * @param bool $isMultiselect
     * @return array
     */
    public function toOptionArray($isMultiselect = false)
    {
        if (!$this->_options) {
            $countriesArray = $this->_countryCollectionFactory->create()->load()->toOptionArray(false);
            $this->_countries = [];
            foreach ($countriesArray as $a) {
                $this->_countries[$a['value']] = $a['label'];
            }

            $countryRegions = [];
            $regionsCollection = $this->_regionCollectionFactory->create()->load();
            foreach ($regionsCollection as $region) {
                $countryRegions[$region->getCountryId()][$region->getId()] = $region->getDefaultName();
            }
            uksort($countryRegions, [$this, 'sortRegionCountries']);

            $this->_options = [];
            foreach ($countryRegions as $countryId => $regions) {
                $regionOptions = [];
                foreach ($regions as $regionId => $regionName) {
                    $regionOptions[] = ['label' => $regionName, 'value' => $regionId];
                }
                $this->_options[] = ['label' => $this->_countries[$countryId], 'value' => $regionOptions];
            }
        }
        $options = $this->_options;
        if (!$isMultiselect) {
            array_unshift($options, ['value' => '', 'label' => '']);
        }

        return $options;
    }

    /**
     * @param string $a
     * @param string $b
     * @return int
     */
    public function sortRegionCountries($a, $b)
    {
        return strcmp($this->_countries[$a], $this->_countries[$b]);
    }
}

Answer 1

Okay finally I found my answer, I create this Modal to get all United States regions app/code/[Vendor]/[Module]/Model/Config/Region/RegionInformationProvider.php

<?php

namespace [Vendor]\[Module]\Model\Config\Region;

class RegionInformationProvider
{
  protected $countryInformationAcquirer;
  protected $addressRepository;

  public function __construct(
      \Magento\Directory\Api\CountryInformationAcquirerInterface $countryInformationAcquirer
  ) {
         $this->countryInformationAcquirer = $countryInformationAcquirer;
  }

  public function toOptionArray()
  {
        $countries = $this->countryInformationAcquirer->getCountriesInfo();
        foreach ($countries as $country) {
            if($country->getId() == 'US'){
                $regions = [];
                if ($availableRegions = $country->getAvailableRegions()) {
                     foreach ($availableRegions as $region) {
                          $regions[] = [
                             'value' => $region->getId(),
                             'label' => $region->getName()
                           ];
                     }
                 }
            }  
        }
        return $regions;

     }
}

In Magento 2 admin panel, go to Stores → Configuration from the left-side navigation panel. Then Goto General section → General → Country options → Choose ‘United States’ in Allowed Countries. Because we only list the allowed countries states in our custom system configuration field.

And your system.xml is something like this:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="carriers" type="text" sortOrder="320" showInDefault="1" showInWebsite="1" showInStore="1">
            <group id="flatrate" translate="label" type="text" sortOrder="0" showInDefault="1" showInWebsite="1" showInStore="1">
                <field id="instorepickup_tax_state" translate="label" type="select" sortOrder="99" showInDefault="1" showInWebsite="1" showInStore="0" >
                    <label>Tax State</label>
                    <source_model>[Vendor]\[Module]\Model\Config\Region\RegionInformationProvider</source_model>
                </field>
            </group>
        </section>
    </system>
</config>